stbt templatematch: Run the image processing algorithm at most once
stbt templatematch: Run the image processing algorithm at most once

By using `detect_match` to process a single frame instead of relying on
a `wait_for_match` timeout of 1 second, we decrease the run time for the
no-match case to 60% of the original run time (on my dual-core 1.8GHz
Intel Core i7).

wmanley/stb-tester,wmanley/stb-tester,LewisHaley/stb-tester,martynjarvis/stb-tester,wmanley/stb-tester,stb-tester/stb-tester,martynjarvis/stb-tester,LewisHaley/stb-tester,LewisHaley/stb-tester,LewisHaley/stb-tester,LewisHaley/stb-tester,wmanley/stb-tester,martynjarvis/stb-tester,martynjarvis/stb-tester,martynjarvis/stb-tester,stb-tester/stb-tester,martynjarvis/stb-tester,stb-tester/stb-tester,stb-tester/stb-tester,martynjarvis/stb-tester,LewisHaley/stb-tester,wmanley/stb-tester,LewisHaley/stb-tester